parse-options API

The parse-options API is used to parse and massage options in git
and to provide a usage help with consistent look.

Basics

The argument vector argv[] may usually contain mandatory or optional
non-option arguments, e.g. a filename or a branch, and options.
Options are optional arguments that start with a dash and
that allow to change the behavior of a command.

There are basically three types of options:
boolean options,
options with (mandatory) arguments and
options with optional arguments
(i.e. a boolean option that can be adjusted).

There are basically two forms of options:
Short options consist of one dash (-) and one alphanumeric
character.
Long options begin with two dashes (\--) and some
alphanumeric characters.

parse_options() will filter out the processed options of argv[] and leave the
non-option arguments in argv[].
argc is updated appropriately because of the assignment.

You can also pass NULL instead of a usage array as the fifth parameter of
parse_options(), to avoid displaying a help screen with usage info and
option list. This should only be done if necessary, e.g. to implement
a limited parser for only a subset of the options that needs to be run
before the full parser, which in turn shows the full help message.

Flags are the bitwise-or of:

PARSE_OPT_KEEP_DASHDASH

Keep the \-- that usually separates options from
non-option arguments.

PARSE_OPT_STOP_AT_NON_OPTION

Usually the whole argument vector is massaged and reordered.
Using this flag, processing is stopped at the first non-option
argument.

PARSE_OPT_KEEP_ARGV0

Keep the first argument, which contains the program name. It’s
removed from argv[] by default.

PARSE_OPT_KEEP_UNKNOWN

Keep unknown arguments instead of erroring out. This doesn’t
work for all combinations of arguments as users might expect
it to do. E.g. if the first argument in --unknown --known
takes a value (which we can’t know), the second one is
mistakenly interpreted as a known option. Similarly, if
PARSE_OPT_STOP_AT_NON_OPTION is set, the second argument in
--unknown value will be mistakenly interpreted as a
non-option, not as a value belonging to the unknown option,
the parser early. That’s why parse_options() errors out if
both options are set.

PARSE_OPT_NO_INTERNAL_HELP

By default, parse_options() handles -h, --help and
--help-all internally, by showing a help screen. This option
turns it off and allows one to add custom handlers for these
options, or to just leave them unknown.

Data Structure

The main data structure is an array of the option struct,
say static struct option builtin_add_options[].
There are some macros to easily define options:

OPT__ABBREV(&int_var)

Add \--abbrev[=<n>].

OPT__DRY_RUN(&int_var)

Add -n, \--dry-run.

OPT__QUIET(&int_var)

Add -q, \--quiet.

OPT__VERBOSE(&int_var)

Add -v, \--verbose.

OPT_GROUP(description)

Start an option group. description is a short string that
describes the group or an empty string.
Start the description with an upper-case letter.

OPT_BOOLEAN(short, long, &int_var, description)

Introduce a boolean option.
int_var is incremented on each use.

OPT_BIT(short, long, &int_var, description, mask)

Introduce a boolean option.
If used, int_var is bitwise-ored with mask.

OPT_NEGBIT(short, long, &int_var, description, mask)

Introduce a boolean option.
If used, int_var is bitwise-anded with the inverted mask.

OPT_SET_INT(short, long, &int_var, description, integer)

Introduce a boolean option.
If used, set int_var to integer.

OPT_SET_PTR(short, long, &ptr_var, description, ptr)

Introduce a boolean option.
If used, set ptr_var to ptr.

OPT_STRING(short, long, &str_var, arg_str, description)

Introduce an option with string argument.
The string argument is put into str_var.

OPT_INTEGER(short, long, &int_var, description)

Introduce an option with integer argument.
The integer is put into int_var.

OPT_DATE(short, long, &int_var, description)

Introduce an option with date argument, see approxidate().
The timestamp is put into int_var.

OPT_CALLBACK(short, long, &var, arg_str, description, func_ptr)

Introduce an option with argument.
The argument will be fed into the function given by func_ptr
and the result will be put into var.
See Option Callbacks below for a more elaborate description.

OPT_FILENAME(short, long, &var, description)

Introduce an option with a filename argument.
The filename will be prefixed by passing the filename along with
the prefix argument of parse_options() to prefix_filename().

OPT_ARGUMENT(long, description)

Introduce a long-option argument that will be kept in argv[].

OPT_NUMBER_CALLBACK(&var, description, func_ptr)

Recognize numerical options like -123 and feed the integer as
if it was an argument to the function given by func_ptr.
The result will be put into var. There can be only one such
option definition. It cannot be negated and it takes no
arguments. Short options that happen to be digits take
precedence over it.

The last element of the array must be OPT_END().

If not stated otherwise, interpret the arguments as follows:

short is a character for the short option
(e.g. \'e\' for -e, use 0 to omit),

long is a string for the long option
(e.g. "example" for \--example, use NULL to omit),

int_var is an integer variable,

str_var is a string variable (char *),

arg_str is the string that is shown as argument
(e.g. "branch" will result in <branch>).
If set to NULL, three dots (...) will be displayed.

description is a short string to describe the effect of the option.
It shall begin with a lower-case letter and a full stop (.) shall be
omitted at the end.

Option Callbacks

The function must be defined in this form:

int func(const struct option *opt, const char *arg, int unset)

The callback mechanism is as follows:

Inside func, the only interesting member of the structure
given by opt is the void pointer opt->value.
\*opt->value will be the value that is saved into var, if you
use OPT_CALLBACK().
For example, do *(unsigned long *)opt->value = 42; to get 42
into an unsigned long variable.

Sophisticated option parsing

If you need, for example, option callbacks with optional arguments
or without arguments at all, or if you need other special cases,
that are not handled by the macros above, you need to specify the
members of the option structure manually.

This is not covered in this document, but well documented
in parse-options.h itself.